In a vehicle equipped with an internal combustion engine such as a diesel engine, an exhaust pipe for guiding exhaust gas from the internal combustion engine to the atmosphere has a diesel particulate filter (DPF), wherein a particulate matter (PM) such as soot contained in the exhaust gas is collected. The DPF, mainly made of ceramic, is a filter that has a number of honeycomb holes (or square holes). In the DPF, the PM adheres to the surface of the honeycomb holes serving as passages for the exhaust gas, whereby the PM is collected.
However, if the DPF is damaged or cracks, then the DPF cannot collect the PM sufficiently, causing the PM to flow out to the downstream side of the DPF and releasing the PM to the atmosphere. In order to prevent this defect, it is desired that a failure in the DPF be detected and handled promptly. In view of such circumstances, in the U.S., it is mandated under the OBD (on-board diagnosis) regulation that a vehicle be equipped with a DPF failure detection monitor.
In conventional technologies, mainly a differential pressure sensor for measuring a difference in pressure in front of and behind a DPF is installed, wherein when an output value of the differential pressure sensor significantly falls below an output range of a normal state of the DPF, it is determined that the DPF has a failure. However, there is a possibility that a method using such a differential pressure sensor has a problem in its detection accuracy and cannot cope with the regulations that become tighter in the future. Therefore, there has been known the technology disclosed in Japanese Patent Application Publication No. 2006-153716 as a PM sensor for detecting the amount of PM in exhaust gas.
However, the PM sensor disclosed in Japanese Patent Application Publication No. 2006-153716 is a fixed equipment used for research and development of internal combustion engines and is not suitable to be installed in a vehicle.
For instance, although a DPF failure detection device to be installed in a vehicle needs to be small, the PM sensor disclosed in Japanese Patent Application Publication No. 2006-153716 consumes a high voltage reaching as much as 2000 to 7000 V and. Such a device tends to be massive in size, weighty, and expensive.
Furthermore, the PM sensor of Japanese Patent Application Publication No. 2006-153716 consumes high voltages and therefore needs to devise safety measures for electrical insulation, again causing a cost increase. An additional concern is that radiation noise is caused due to the consumption of high voltages.